This invention relates to a hydraulic flushing system and method of flushing a downhole control line in an underwater, e.g. subsea, hydrocarbon well facility.
Surface controlled sub-surface safety valves (SCSSVs) on production fluid wells are controlled by high pressure hydraulic fluid switched through a directional control valve (DCV). After a period of time, the hydraulic fluid can deteriorate or become contaminated resulting in possible failure of the SCSSV to operate when required. In order to prevent this problem, well operators insist that the design of the hydraulic system allows for flushing of the hydraulic fluid from the hydraulic control lines right down to the SCSSV itself. However, existing methods of achieving this involve a second ‘flushing’ DCV and typically venting of the flushed hydraulic fluid to the sea. This creates two new problems: a) the hydraulic fluid can be contaminated with particles which can lodge in the flushing DCV causing it to fail to close resulting in total failure of the SCSSV control; and b) the hydraulic fluid, normally not a pollutant when vented to sea, can be contaminated with downhole fluids including hydrocarbons, which cause the hydraulic fluid to become a pollutant. The invention aims to overcome some of the above problems.
FIG. 1 illustrates a typical existing method of flushing a hydraulic line to a SCSSV 1 of a subsea hydrocarbon well facility. High pressure (HP) hydraulic fluid, typically sourced via an umbilical from a surface platform, is fed via a line 2 and a subsea Xmas tree 3, to a subsea control module (SCM) 4, housed in the tree. Reference numeral 5 designates a base plate for the SCM 4.
During normal operation, the SCSSV 1, is opened and closed by operating a DCV 6, whilst a ‘flushing’ DCV 7 remains closed. In order to flush a hydraulic downhole control line 8 of the SCSSV 1, the DCV 6 is closed, the DCV 7 is opened followed by the opening of DCV 6, allowing control fluid to flow through the hydraulic downhole control line 8, to the SCSSV 1, and then back up a second control line 9 (acting as a purge or flushing line), through the DCV 7, through a flow transmitter 10, if fitted (this component may be omitted in practice), a metallic check valve 11, and finally a seawater check valve 12 before being vented to sea. The orifice in a DCV such as DCV 7 in this system is typically only 3 millimeters in diameter, and is thus prone to blockage from contaminating particles.
As previously described, prior art systems such as the one shown in FIG. 1 suffer from the problem of potential particle and chemical contaminated fluid being flushed into the sea.